Method and apparatus for casting of metal articles using external pressure

Abstract

A method for casting of metal articles using external pressure and having particular application to lost foam casting of metal articles. A polymeric foam pattern having a configuration corresponding to an article to be cast is placed in an outer flask and the pattern is connected through a polymeric foam gating system to a pouring cup located at the upper end of the flask. The pouring cup has a volume equal to 5% to 75% of the combined volume of the gating system and the pattern. A finely divided inert material, such as sand, is placed in the flask surrounding the pattern and fills the internal cavities within the pattern. The flask containing the pattern is then positioned in an outer pressure vessel having a removable lid and a molten metal is fed into the pouring cup. The lid on the pressure vessel is closed and an external gaseous pressure is applied to the molten metal in the pouring cup as the molten metal feeds through the gating system to the pattern and progressively decomposes the polymeric foam material. The gaseous products of decomposition passing into the interstices of the sand and the molten metal filling the void created by decomposition of the foam. By applying pressure to the molten metal during filling, the molten metal front is more stable and fewer casting defects arise.

We claim:  
     
       1. A method of lost foam casting, comprising the steps of positioning a polymeric foam pattern corresponding in configuration to an article to be cast in a flask, connecting the pattern to a polymeric foam gating system located in the flask, introducing a finely divided inert material in the flask to surround the pattern and the gating system and fill internal cavities in the pattern, placing the flask and the pattern in an outer pressure vessel, positioning a pouring cup in the pressure vessel and connecting said pouring cup with said gating system, pouring a molten metal into the pouring cup, the heat of said molten metal acting to decompose the gating system and pattern with the molten metal filling the void created by decomposition of the polymeric foam material and the products of decomposition passing into the interstices of the finely divided material, sealing the pressure vessel with the pouring cup retained within said vessel, applying an external pressure in the range of 5 to 60 atmospheres to the molten metal in the pouring cup and to the finely divided material in the flask after sealing the vessel and before the molten metal has fully decomposed the pattern, maintaining said pressure on said molten metal until said molten metal fully decomposes the pattern, and continuing said pressure during solidification of said molten metal to produce a cast article corresponding in configuration to the pattern. 
     
     
       2. The method of  claim 1 , wherein said molten metal is selected from the group consisting of an aluminum alloy, a magnesium alloy, cast iron and a stainless steel alloy. 
     
     
       3. The method of  claim 1 , wherein said pattern comprises a component of an internal combustion engine. 
     
     
       4. The method of  claim 1 , where in the pouring cup has a volume equal to 15% to 75% of the combined volume of the gating system and said pattern. 
     
     
       5. The method of  claim 1 , wherein th e flask is composed of metal and the pouring cup is composed of ceramic material. 
     
     
       6. The method of  claim 1 , and including the step of restricting the flow of molten metal from the pouring cup to the pattern to ensure that said pressure is applied to the molten metal before the pattern is fully decomposed. 
     
     
       7. An apparatus for lost foam casting of metal articles, comprising a pressure vessel having a removable lid, a flask removably contained and sealed within the pressure vessel, a polymeric foam pattern corresponding in configuration to an article to be cast and disposed in the flask, a finely divided inert material disposed in the flask and surrounding the pattern and filling voids in the pattern, a pouring cup, having a volume equal to 15% to 75% of the combined volume of a gating system and the pattern, disposed in the pressure vessel within the lid, said polymeric foam gating system connecting the pouring cup with the pattern, a molten metal contained in the cup, the heat of the molten metal acting to ablate the gating system and pattern with the molten metal filling the void created by ablation of the gating system and pattern and the products of decomposition being trapped in the interstices of the finely divided material, a means for applying pressure to the molten metal in the pouring cup and to said inert material in the flask before filling of the void with said molten metal is complete, and a means for restricting the flow of molten metal from the pouring cup to the pattern to ensure that the pressure is applied to the molten metal before the pattern is fully decomposed.